Dropped packets because of "Invalid TCP Flag"

This article describes how to workaround the drop "(Invalid TCP Flag(#2)), Module Id: 25(network)" due to network issues.

CAUTION: This KB only shows a possible workaround for the issue however most of the drops due to Invalid TCP Flags are related to network issues and they should be analysed and corrected.

If the TCP Flags behavior is wrong, following this KB article will not bring any improvement. 

• Packets may be perceived as having Invalid TCP flag if packets with SYN+ACK+PSH, instead of SYN+ACK, are received.
• Packets may get to the SonicWall with incorrect sequence numbers due to 3rd party issues or source configuration (i.e. sequence number randomization).

Resolution for SonicOS 7.X

This release includes significant user interface changes and many new features that are different from the SonicOS 6.5 and earlier firmware. The below resolution is for customers using SonicOS 7.X firmware.

• Enable Fix/ignore malformed TCP headers and disable Enable TCP sequence number randomization in the internal settings page.

1. Enter the internal settings page by entering "https://<IP ADDRESS>/sonicui/7/m/Mgmt/settings/diag" in the address bar.
2. Click on Internal Settings.
   
   
   
   
   
   
3. Enable Fix/ignore malformed TCP headers and disable Enable TCP sequence number randomization.
   
   

• If the drop is related to incorrect sequence number, you might disable Enforce strict TCP compliance with RFC 793 and RFC 1122 under Network tab Firewall | Flood Protection | TCP as this could be related to the sending device not fully complying with TCP standards.
  
  

Resolution for SonicOS 6.5

This release includes significant user interface changes and many new features that are different from the SonicOS 6.2 and earlier firmware. The below resolution is for customers using SonicOS 6.5 firmware.

 NOTE: Invalid TCP Flag drops are usually related to a 3rd party issue as the packets are arriving to the SonicWall with a wrong sequence number or in wrong order.

• Enable Fix/ignore malformed TCP headers and disable Enable TCP sequence number randomization in the internal settings page.
  1. Enter the internal settings page by entering diag.html in the address bar in place of main.html.
  2. Click on Internal Settings.
     

     

     

  3. Enable Fix/ignore malformed TCP headers and disable Enable TCP sequence number randomization.
     

• If the drop is related to incorrect sequence number, you might disable Enforce strict TCP compliance with RFC 793 and RFC 1122 from Firewall Settings | Flood Protection as this could be related to the sending device not fully complying with TCP standards.

Resolution for SonicOS 6.2 and Below

The below resolution is for customers using SonicOS 6.2 and earlier firmware. For firewalls that are generation 6 and newer we suggest to upgrade to the latest general release of SonicOS 6.5 firmware.

Here's some workarounds for this issue.

NOTE: Invalid TCP Flag drops are usually related to a 3rd party issue as the packets are arriving to the SonicWall with a wrong sequence number or in wrong order.

• Enable “Fix/ignore malformed TCP headers“ and disable “Enable TCP sequence number randomization” in the internal settings page.
  1. Enter the internal settings page by entering diag.html in the address bar in place of main.html.
  2. Click Internal Settings.
     

     

     

  3. Enable Fix/ignore malformed TCP headers and disable Enable TCP sequence number randomization.
      

• If the drop is related to incorrect sequence number, you might disable Enforce strict TCP compliance with RFC 793 and RFC 1122 from Firewall Settings | Flood Protection as this could be related to the sending device not fully complying with TCP standards.